The current emphasis on fuel economy in the design of power plants for automotive application has resulted in efforts to improve the performance of relatively small displacement engines so as to provide adequate performance characteristics under high torque demand conditions while enjoying the relatively moderate fuel usage associated with such small displacement engines.
Supercharges have long been utilized to boost the power output of internal combustion piston engines of both spark and compression ignition. One common supercharger arrangement currently in use is the turbocharger, in which the engine exhaust flow is utilized to drive an exhaust turbine, which in turn drives a compressor turbine to provide supercharged air flow from the turbine compressor to the engine intake manifold. Such turbochargers of necessity must run at relatively high rotative speeds, i.e., on the order of 80,000 to 100,000 rpm, which requires relatively costly construction.
Furthermore, the nature of the exhaust gas flow and the turbine drive arrangements is such that the supercharging flow increases exponentially and creates relatively inadequate boost pressures at low engine speeds and excessive boost pressures at relatively high engine speed in the absence of control arrangements for reducing flow.
Thus, the torque available at low speeds is not adequate for optimal performance characteristics and in addition requires an arrangement for bypassing of the exhaust flow from the turbine at relatively high engine speeds or some other means to eliminate the excess boosting of pressure which would otherwise occur.
On the other hand, such units do afford the advantages of relatively smooth transitions from natural aspiration to supercharged operation and utilizing a driving force hot exhaust gas, the energy of which would otherwise be largely wasted. In addition, these devices are sensitive to back pressure in the development of output flow and can operate under relatively high back pressure without corresponding decreases in efficiency.
Mechanical engine driven blower arrangements have also been utilized in the past, some of which were nonpositive displacement turbine-type compressors which, as in the above designs, do not provide adequate flow at low engine speeds in order to satisfy the aforementioned desired performance criteria.
Positive displacement type air pumps have been employed which in many cases were driven by the engine at all times to provide supercharging under all engine operating conditions. This compromises the potential economy of operation of low displacement engines, inasmuch as the economy of operation of such engines which are constantly supercharged is not improved over larger displacement engines exhibiting similar performance characteristics.
There has thus been provided arrangements in enabling the operating of the mechanically driven blowers to be activated only in a predetermined engine operating range by means of on/off clutches and the like.
Difficulties are involved in the use of an on/off mechanically driven positive displacement pump, particularly of a high volume output which is required to enable high torque to be generated at relatively low engine speed.
One difficulty is in the transition period when the clutch is first engaged to initiate blower operation. The engine goes from a situation of natural aspiration to a situation in which there is a large stepped increase in the intake air pressure which causes an objectionable torque surge particularly for larger displacement engines.
Such torque surge is not present in the turbocharger design since the nonpositive flow characteristics of turbines enables a relatively smoother transition and also these devices are often operated throughout the range of engine operating conditions.
That is, if the air pump is not provided with a variable speed drive upon activation of the air pump, a relatively large sudden increase in the intake pressure to the engine results, i.e., for example, a six psi increase over atmospheric pressure. For a spark ignition engine, such pressure corresponds substantially directly to the torque output of the engine, i.e., the increase intake pressure produced is an increased air flow through the carburetor, in turn inducing a corresponding increase in the mass of fuel-air mixture into the engine cylinders and a corresponding increase in torque output of the engine.
It can be seen that if the air pump is activated with a stepped increase in air flow to the engine, the corresponding increase in torque produces the sudden increase in torque noted above.
A further difficulty in the operation of positive displacement pumps is encountered during engine operation at supercharger boost pressures below peak boost pressure. Such conditions occur for example while ascending a grade at a speed which does not require peak supercharged conditions but does require the operation of the supercharger. If the throttle is not fully open, the throttle plate represents a pressure restriction downstream of the supercharger creating a back pressure on the supercharger leading to increased horsepower consumption to drive the air pump and the resulting inefficient and unnecessary supercharging operation. The high pressure applied to the carburetor openings upstream of the throttle plate can also cause flooding and wastage of fuel.
In U.S. Pat. No. 2,486,047 to Marinelli, there is disclosed an arrangement for controlling the power applied to the blower in accordance with the differential pressure across the engine throttle plate in order to vary the supercharging activity to maintain a constant pressure differential across the throttle plate. While potentially offering a way to alleviate the aforementioned difficulties, this arrangement involves the use of a variable speed drive which greatly increases the expense of the unit and renders it more or less impractical for such high volume automotive applications. Furthermore, this particular arrangement employs a nonpositive displacement blower which has a tendency to produce inadequate boost pressure and torque at relatively low engine speeds, as with turbochargers.
One type of positive displacement air pump which has been employed in the past in these applications is a vane pump of the general type including a plurality of radially extending vanes which are carried by a cylindrical rotor, which rotor is rotated within a housing chamber about an axis eccentric to that of the housing chamber and to a fixed offset axis shaft upon which are journalled the vane hubs.
As noted, the vanes disposed within a housing having a cylindrical section configuration, the center of which is also offset from the rotor axis, are such that rotation of the rotor within the housing produces increasing and decreasing volume working chambers intermediate the radial vanes. By providing suitably located inlet and exhaust ports in the housing chamber wall, a simple positive displacement air pump has been heretofore provided. The arrangement of a fixed offset axis shaft upon which the vane hubs are rotatably mounted has in the past generally been cantilevered, requiring a relatively large diameter shaft being employed together with relatively expensive combined radial and thrust bearing arrangements for each vane. Since such air pump is the major element in the expense of such unit, it of course would be advantageous if the cost of such unit could be reduced over such heretofore known designs for such high volume designs suited to the application described above.
A further difficulty is in the design of the unit and the air pump itself in that the design of the positive displacement generally requires very effective sealing of the moving elements as opposed to the nonexistent sealing of turbine blades with such sealing means also being required to be relatively durable for automotive applications.
Accordingly, it is an object of the present invention to provide a supercharging system for internal combustion engines which can provide high volume boost flow and pressure at relatively low engine speeds and which is operational only under high torque demand engine operating conditions so as to enable economy operation of the engine under low torque demand conditions, while providing improved performance characteristics during conditions of relatively high torque demand.
It is a further object of the present invention to provide such supercharging system in which the activation of the supercharger system does not result in a sudden increase in torque output of the engine due to the rapid increase in air pressure to the carburetor.
It is yet another object of the present invention to provide such supercharger system which does not involve the use of variable speed drives in order to control the output of the supercharger air pump.
It is still another object of the present invention to provide such supercharger system employing a relatively high volume positive displacement air pump in which the output flow is varied without significant throttling of the air flow into the engine in order to enable relatively high efficiency driving of the supercharger air pump.
It is another object of the present invention to provide an air pump for such supercharger system which provides relatively high pressure supercharging at low engine speeds in order to enable adequate engine torque increases at such low engine speeds.
It is still another object of the present invention to provide an improved version of a vane type air pump suitable for this application of the type including a plurality of radially extending vanes disposed within a cylindrical chamber formed in a housing, which vanes are rotated by an eccentrically journalled cylindrical rotor, carrying seals through which the vanes pass. The vanes are rotatably mounted on a fixed shaft offset from the rotor axis but on the chamber axis, in which simplified bearing arrangements are provided.
It is still another object of the present invention to provide such vane type air pump in which the vanes are supported by the offset axis fixed shaft securely such as to enable the close vane-to-housing clearance to yield a high efficiency pumping action.